Tracheal intubation is the placement of a flexible plastic tube into the trachea to protect the patient's airway and provide a means of mechanical ventilation. The most common tracheal intubation is orotracheal intubation where, with the assistance of a laryngoscope, an endrotracheal tube is passed through the mouth, larynx, and vocal cords, into the trachea. A bulb is then inflated near the distal tip of the tube to help secure it in place and protect the airway from blood, vomit, and secretions. Orotracheal intubation is often performed as needed in medical procedures by anaesthesiologists and other specialists, such as intensivists, pulmonologists, and emergency room doctors.
Maintenance of airway patency is a primary responsibility of anesthesiologists. Interruption of gas exchange, even for several minutes, can result in catastrophic outcomes such as brain damage or death. It has been reported that the vast majority (85%) of airway-related events involve brain damage or death, and as many as one third of deaths attributable solely to anesthesia have been related to inability to maintain a patent airway. The difficulty of achieving a patent airway varies with anatomic and other individual patient factors, and identification of the patient with a difficult airway is vital in planning anesthetic management so that orotracheal intubation and positive pressure ventilation can be achieved safely. Several clinical criteria can be routinely assessed on patients prior to anesthesia including mouth opening distance, Mallampati classification, neck mobility, ability to prognath, thyromental distance, body weight, and previous history of difficult intubation.
Accurate preoperative prediction of potential difficulty with intubation can help reduce the incidence of catastrophic complications by alerting anesthesia personnel to take additional precautions before beginning anesthesia and establishing an artificial airway. In addition, more accurate prediction of difficulty with intubation can potentially reduce the frequency of unnecessary maneuvers (for instance, awake intubation) related to false positive predictions.
Several evaluation criteria have been proposed. Recently, Janssens et al have proposed a quantitative evaluation approach using airway difficulty score (ADS) as shown in the table below. Airway Difficulty Score represent the sum of the points for five criteria of difficult intubation. As shown, the total score can vary from 5 to 15, and if score is higher or equal to 8, ventilation and/or intubation are considered likely to be difficult.
Airway Difficulty Score (ADS)Score123Thyromental distance>6 cm5-6 cm<5 cmMallampati classClass IClass IIClass III & IVMouth opening 4 cm2-3 cm 1 cmNeck mobilityNormal (≧35°)ReducedFixed flexionUpper incisorsAbsentNormalProminentIf score ≧8, ventilation and/or intubation likely to be difficult (Janssens et al., European Journal of Anaesthesiology, 2001, 18, 3-12)
In addition to the above five parameter, mandibular angle distance has also been considered important in evaluation of airway. Among these tests, Mallampati class and upper incisors can be examined by visual observation. However, determination of mouth opening distance, thyromental distance, mandibular angle distance, and neck mobility of a patient requires using measurement equipments.
Currently, limited tools are available for making the above described distance and angular measurements. Doctors use ruler, measuring tape, paquimeter, and also commonly use their fingers or hands, to make the distance measurements. Moreover, there is no specific device available for clinical measurement of the neck mobility angle. This renders the measurements difficult and less accurate, considering the subject of the measurements and unnatural positions that the patient is in. Lack of accuracy in the measurements may result in life threatening consequences, for example when a false negative is reported from the measurements.
Furthermore, for distance and angular measurements at least two different measuring devices are used. As such, doctors need to carry, or have access to, more than one device for the required measurements. On the other hand, considering the subject and environment of the measurements, the devices used should be either disposable or can be sterilized if they are intended to be used repetitively. It is costly to supply and maintain multiple disposable measurement devices.
Therefore, there exists a strong need for a measurement device that is designated and particularly suitable for providing accurate airway evaluation for orotracheal intubation, and an integrated measurement device that can be used for both distance and angular measurements for airway evaluation. It is further desirable to have a low cost disposable device that can be conveniently carried by doctors or made readily available in medical facilities.